Information record with a thick overcoat

ABSTRACT

The invention is an improved optical recording medium and a method for making same in which the substrate has a raised center and peripheral portions. The sides of the raised portions and the upper surface of a light absorptive layer, which overlies the unraised portion, form a mold which is filled with the overcoat material.

This is a division of application Ser. No. 108,030, filed Dec. 28, 1979,now U.S. Pat. No. 4,340,959.

The invention is an optical recording medium having a thick overcoatlayer and a method for making same.

BACKGROUND OF THE INVENTION

Spong, U.S. Pat. No. 4,097,895 issued June 27, 1978 and incorporatedherein by reference, has disclosed an ablative optical recording mediumfor use in an optical recording system. The medium comprises a lightreflective material which is coated with a layer of a light absorptiveorganic material. The thickness of the light absorptive layer is chosenso that the reflectivity of the recording medium is reduced. A focused,modulated light beam, such as a light beam from an argon ion laser, whendirected at the recording medium vaporizes or ablates the lightabsorptive layer leaving an opening in this layer and exposing the lightreflecting material. Bell, U.S. Pat. No. 4,216,501, which isincorporated herein by reference has disclosed an improved ablativetrilayer optical recording medium for use in the Spong optical recordingsystem. The trilayer optical recording medium comprises a lightreflective layer, a light transmissive layer overlying the lightreflective layer, and a light absorptive layer overlying the lighttransmissive layer. The thickness of the light absorptive layer is sorelated to the thickness of the light transmissive layer and the opticalconstants of the light reflective, transmissive and absorptive layers soas to reduce the optical reflectivity of the recording medium. Afocused, modulated light beam ablates or melts the light absorptivelayer thus exposing the underlying light reflective layer through thelight transmissive layer.

The reflectivity in the area of the opening in the light absorptivelayer is essentially that of a light reflective layer and is muchgreater than that of the surrounding, unexposed region. During readoutthis difference in reflectivity is detected optically and converted intoan electrical signal representative of the recorded information.

To eliminate or reduce signal defects or dropouts caused by surface dustwhich precipitates onto the medium from the environment, a thickovercoat is applied to the light absorptive layer. Dust particles andother surface contaminants which settle on the upper surface of theovercoat layer are thus far removed from the focal plane of therecording lens so that their effect on the recording or playback signalis considerably reduced. Bloom et al, in U.S. Pat. No. 4,315,269,incorporated herein by reference, disclose a thick overcoat with apreferred range of thicknesses from about 0.05 mm to about 1 mm anddescribe a thick overcoat about 0.08 mm thick formed by spinningtechniques. Bell et al, U.S. Pat. No. 4,101,907 issued July 18, 1978 andincorporated herein by reference, disclose an overcoat structureconsisting of a thin layer which forms a chemical and thermal barrierbetween the light absorbing layer and a thick overcoat layer overlyingthe thin overcoat. The thin overcoat is typically 0.0003 mm thick and istypically formed by evaporation of silicon dioxide. The thick overcoatis typically about 0.1 mm thick and is formed by spinning techniques.

As the thickness of the overcoat layer is increased it becomes moredifficult to obtain an overcoat layer by spinning techniques which has aradially uniform thickness because of the increased viscosity of theovercoat material and the slower spinning speeds required. The optimalthickness of the overcoat layer is a balance between the maximumthickness to provide maximum immunity to surface contamination and aminimum thickness to provide a uniform thickness, in order to reduceoptical thickness variations, and to minimize manufacturing costs andtime. Overcoat layers about 0.18 mm thick are useful since thisthickness corresponds to the standard cover glass correction built intocommercially available microscope objectives. Thus, it would bedesirable to have an alternative method to spinning for forming thickovercoat layers which are uniform in their thickness.

SUMMARY OF THE INVENTION

An overcoated, optical recording medium comprises a substrate, a lightreflective layer overlying the substrate, a light absorptive layeroverlying the light reflective layer, and a thick overcoat layeroverlying the light absorptive layer. The invention is an improvedoptical recording medium and information record and a method of makingsaid medium and record wherein the improvement comprises a substratehaving a major surface which has a center portion and a portionextending about its periphery which are raised above the major surface.The light reflective and light absorptive layers overlie at least aportion of that part of the major surface which is not raised. The uppersurface of the light absorbing layer and the sides of the raised centraland peripheral portions form a mold in which the overcoat material canbe cast and then cured or hardened.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic illustration of a cross-sectional view of anovercoated optical recording medium of the invention.

FIG. 2 is a schematic illustration of a cross-sectional view of anovercoated optical recording medium of the invention having a trilayeroptical recording structure.

FIG. 3 is a schematic illustration of a cross-sectional view of a secondembodiment of the overcoated optical recording medium of the inventionwherein openings have been formed in the raised peripheral portion ofthe substrate.

FIG. 4 is a schematic illustration of a cross-sectional view of aportion of an overcoated information record of the invention havinginformation recorded therein.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic illustration of a cross-sectional view of animproved optical recording medium 10 of the invention which comprises asubstrate 12 having a center portion 14 and a peripheral portion 16,extending about the substrate, which are raised above a major surface 18of the substrate 12. The raised center portion 14 may have an opening 15extending therethrough to the substrate surface opposed to the majorsurface 18. A light reflective layer 20 overlies at least a portion ofthe major surface 18 of the substrate 12. A light absorptive layer 22overlies the light reflective layer 20 over at least a portion of themajor surface 18 which is not raised. An overcoat layer 24 overlies theupper surface 26 of the light absorptive layer 22. Overcoat layer 24 mayhave a thickness such that its upper surface is co-extensive with theupper surface 28 of the raised center portion 14 or the upper surface 30of the raised peripheral portion 16 or a thickness less than thismagnitude.

FIG. 2 is a schematic illustration of a cross-sectional view of anovercoated optical recording medium 40. The identification of theelements of the medium 40 correspond to those of FIG. 1 except that thelight absorptive layer 22 is comprised of two layers: a spacer layer 42and an absorptive layer 44.

FIG. 3 is a schematic illustration of a cross-sectional view of a secondembodiment 50 of the improved overcoated optical recording medium of theinvention. The identification of the elements of the second embodiment50 are the same as those for the first embodiment 10 shown in FIG. 1except for the openings or slots 52 in the raised peripheral portion 16of the substrate 12.

FIG. 4 is an illustrative embodiment of an overcoated information record60 of the invention with information recorded therein. Theidentification of the elements of the information record 60 correspondto those of the recording medium 10 of FIG. 1. The FIGURE schematicallyillustrates a section of the recording medium in which an informationtrack has been recorded in the form of a series of openings 62 and lightabsorptive layer 22. Typically, information is recorded in the medium byvarying the length of the openings 62 and of the unexposed areas 64 ofthe light absorptive layer 22 between the openings 62 along thedirection of a track. The length of the openings 62 are determined bythe length of the time that the recording medium is exposed to therecording light beam and the speed at which the recording medium ismoving through the focal plane of the recording light beam.

The substrate 12 may be comprised of a metal, which is machined, or aplastic material, such as polyvinylchloride, which is injection orcompression molded, into the shape shown in FIGS. 1, 2 and 3. The raisedcenter portion 14 and the raised peripheral portion 16 extend a distanceabove the major surface 18 at least as great as the sum of thethicknesses of the light reflective and absorptive layers, and theovercoat layer 24 plus any additional layers. Preferably the height ofthe raised portions 14 and 16 above the major surface 18 are equal. Acenter opening 15 extending through the raised center portion formounting the substrate on the spindle may be present.

In the second embodiment of the invention 50 there are shown radiallydirected openings or slots 52 extending through the raised portion 16 ofthe substrate 12. The radially directed openings 52 are located adistance above the major surface 18 equal to the desired height of thecombination of the light reflective, absorptive and overcoat layersabove the major surface 18. The function of these openings will bedescribed more fully below.

The thickness of the substrate 12 need only be sufficient to support theremainder of the structure.

Any roughness of the surface 18 of the substrate 12 on the scale of thefocused light beam diameter will produce noise in the signal channelduring readout. The interposition of a non-conformal coating of aplastic material such as an epoxy resin between the major surface 18 andthe light reflective layer 20 will produce a microscopically smoothsurface and eliminate this noise source.

The light reflective layer 20 reflects a substantial fraction of thelight incident at the recording or readout wavelength, preferably atleast 30 percent, and is typically formed of a metal such as aluminum orgold which exhibits high reflectivity. The reflective layer 20, which istypically about 30 to 60 nanometers thick, may be deposited on thesurface 18 of the substrate 12 using vacuum evaporation techniques.

The light absorptive layer must be absorbing at the wavelength used forrecording and readout and must form a smooth, amorphous, coherent,continuous, optically clear film. The thickness of the light absorptivelayer may be chosen such that the reflectivity of the recording mediumis reduced. Further, the light absorptive layer should be readilyablatable at low temperatures to form clearly defined openings.

The light absorptive layer 22 may be comprised of an organic dyestuffsuch as 4-phenylazo-1-naphthylamine, as disclosed by Bloom et al in U.S.Pat. No. 4,023,185 issued May 10, 1977 and incorporated herein byreference, phthallocyanine dyes including lead phthallocyanine,chloroaluminum phthallocyanine, vanadyl phthallocyanine, stannicphthallocyanine, and Pt-bis(dithio-αdiketone) complexes having phenyl orsubstituted phenyl groups.

Alternatively, a light transmissive layer may be interposed between thelight reflective layer and the light absorptive layer as shown in FIG.2. Suitable light transmissive materials include silicon dioxide,aluminum oxide, magnesium fluoride and lead fluoride, and plasticmaterials such as fluorocarbon and hydrocarbon polymers. The thicknessof the light transmissive layer is at least 10 nanometers, typicallybetween about 10 nanometers and 500 nanometers and preferably betweenabout 30 nanometers and 100 nanometers. The light absorptive layer istypically a metal between about 2 nanometers and about 25 nanometersthick. The metal may be selected from a group consisting of titanium,platinum, rhodium, gold, nickel, chromium, manganese, vanadium, orselenium, tellurium or allows thereof.

Preferably the reflectivity of the reflective layer 20 and theabsorptive layer 22 is less than 30 percent.

The overcoat layer 26 should be substantially transmissive andnon-scattering at the wavelength used for recording and readout andshould be stable under ambient conditions. When information is recordedin the absorptive layer 22 the overcoat should allow formation of theinformation elements and readout without substantially affectingreproducibility of the recorded information. Silicone resins such asGeneral Electric's RTV 615 and RTV 602 and Dow Corning's Sylgard 184form suitable overcoat materials. Room temperature or radiation curedepoxy resins are also suitable overcoat materials.

The overcoat layer may be between about 0.05 millimeters and about 1millimeter thick, with the effectiveness of the overcoat layerincreasing with increasing thickness. In the optical recording medium ofthe invention the thickness of the overcoat layer depends upon theamount of material dispensed into the mold formed by the sides of thecenter and peripheral raised portions until the level of such materialreaches the top of these portions or the level of the openings in theraised portions, as shown in FIG. 3.

A barrier layer which is either thermally insulating or chemicallyunreactive or both, may be interposed between the light absorptive layer22 and the overcoat layer 24. Bell et al, U.S. Pat. No. 4,101,907,incorporated herein by reference, describe such barrier layers.

Attempts to overcoat a substrate as shown in FIGS. 1 or 3 using aspinning technique can result in a non-uniform overcoat of the opticallysensitive area. In particular, as the overcoat material spreads radiallyoutward, it piles up against the inner wall of the raised peripheralportion producing a radially increasing thickness of the overcoat layerwith the thinnest overcoat near the side of the raised center portion.

A uniform overcoat thickness is required for the present applicationsince the light beam focus servomechanism responds to differences in thespacing between the focal plane of the light beam and the lightreflective layer. This difference is typically detected as a change inthe optical path length of the light beam or as a change in the value ofa capacitor in which the overcoat layer forms part of the dielectric.Since the optical and low frequency dielectric constants of the overcoatmaterial differ from those of air, variations in the thickness of theovercoat will lead to an error in the determination of the position ofthe light absorptive layer. This will result in a light beam not focusedon the light absorptive layer which leads to decreased recordingsensitivity and frequency response of the recording medium.

The optical recording medium disclosed herein uses the raised center andperipheral portions and the major surface, coated with the lightreflective and absorptive layers, as the three sides of a mold intowhich the overcoat material is cast. In the embodiment shown in FIG. 1,the thickness of the resultant overcoat is controlled by the amount ofthe material dispensed onto the major surface until the overcoatthickness equals the height of the raised portions. Thus, the use of theimproved substrate of the invention provides a method by which anovercoated optical recording medium with a uniform and reproduciblethickness can be fabricated. The thickness of the overcoat layer issimply controlled by controlling the amount of material dispensed intothe mold up to the point where the material spills over the raisedperipheral portion.

The method of fabricating an overcoated optical recording mediumcomprises the following steps: (a) forming a substrate having raisedcentral and peripheral portions; (b) forming the light reflective layeron the unraised portion of the major surface; (c) forming a lightabsorptive layer overlying the light reflective layer; (d) forming theovercoat layer by dispensing the desired amount of the overcoat materialinto the mold formed by the side walls of the raised portions and thelight absorptive layer; (e) curing or hardening the overcoat material. Alight transmissive layer may be interposed between the light reflectiveand absorptive layers, if desired. A chemical and thermal barrier layermay be interposed between the light absorptive and overcoat layers, ifdesired.

In the second embodiment as shown in FIG. 3, the thickness of theovercoat depends upon the amount of material dispensed upon the majorsurface until the material fills this mold to the height of the openingsor slots in the peripheral raised portion. Any excess material will thenspill out through the slots. The advantage of the second embodiment isthat the control of maximum thickness which the first embodimentprovides is retained while the upper surfaces of the raised portions canbe used to provide further protection, when the recording media arestacked, if this should be desired.

After the mold is filled with the overcoat material it must be kept in alevel position until the overcoat material cures or hardens, otherwise aside-to-side gradient in the overcoat thickness will result. Analternative method of fabricating the overcoat layer which minimizesthis problem is to fill the mold to the height of the raised portionsand then clamp a plate having a flat surface, which may be coated with asuitable mold release agent, against the upper surfaces 28 and 30 of theraised portions before curing or hardening. This plate is then removedafter the curing or hardening step.

Another alternative method is to use an approach similar to thatdisclosed by Jebens, U.S. Pat. No. 4,170,616 issued Oct. 9, 1979 andincorporated herein by reference, wherein a vacuum casting method wasused to fabricate a thin Fresnel lens on a glass substrate. To fabricatean overcoated optical recording medium, the flat surface of a plate,which may be coated with a mold release agent, is placed against theupper surfaces 28 and 30 of the raised portions of the substrate. Theambient pressure in the chamber formed by the substrate and the plate isthen reduced using vacuum means. The chamber is then filled with theovercoat material, preferably using the same tube as was used for thepressure reduction. Since the pressure differential between the chamberand the ambient becomes zero when the chamber is filled, distortions ofthe substrate or plate which might cause variations in the overcoatthickness are eliminated. After the overcoat material is cured orhardened the flat plate is removed.

I claim:
 1. In an information record, having an information tracktherein, for use in an optical recording and readout system employing alight beam of a certain wavelength which comprises:a substrate having amajor surface; a light reflective layer, which reflects a substantialportion of light incident thereon at said wavelength, overlying at leasta portion of said major surface; a light absorptive layer, which absorbslight at said wavelength, overlying said light reflective layer; anovercoat layer, which is substantially transmissive of light at saidwavelength, overlying said light absorptive layer; wherein saidinformation track comprises a sequence of openings in said absorptivelayer with variations in either or both the length of the openings alongthe track and the spacing between successive openings beingrepresentative of the recorded information; the improvement whichcomprises a substrate having a major surface which has a center portionand a portion extending about said substrate periphery raised a distanceabove said major surface, wherein said light reflective layer, and saidlight absorptive layer overlie at least a portion of the major surfacewhich is not raised and said overcoat layer overlies said major surfacewhich is not raised.
 2. An information record according to claim 1wherein said raised peripheral portions contain one or more openingsextending radially therethrough and extending upwards a distance from apoint above the surface of said light absorptive layer.
 3. Aninformation record according to claim 1 wherein the height of saidraised portions above said major surface of said substrate is betweenabout 0.05 millimeter and about 1 millimeter.
 4. An information recordaccording to claim 1 wherein said light reflective layer is a metalbetween about 30 nanometers and 60 nanometers thick.
 5. An informationrecord according to claim 4 wherein said light absorptive layer is anorganic material whose thickness is adjusted such that the reflectivityfrom said recording medium is reduced.
 6. An information recordaccording to claim 4 wherein a light transmissive layer, at least 10nanometers thick, is interposed between said light reflective layer anda light absorptive layer wherein the optical constants of saidreflective, transmissive and absorptive layers and the thicknesses ofsaid transmissive and absorptive layers are such that the reflectivityof said recording medium at said wavelength is less than about 0.3. 7.An information record according to claim 6 wherein the thickness of saidtransmissive layer is between about 10 nanometers and 500 nanometersthick and said absorptive layer is between about 2 nanometers and 25nanometers thick.
 8. An information record according to claim 6 whereinsaid light transmissive layer is comprised of a material selected from agroup consisting of silicon dioxide, aluminum oxide, magnesium fluoride,lead fluoride and plastic materials and wherein said light absorbinglayer is comprised of a material selected from a group consisting oftitanium, platinum, rhodium, gold, nickel, chromium, bismuth, manganese,vanadium, selenium, and alloys thereof, tellurium and alloys thereof.